Generally, when a cap is attached to a container opening portion by capping, the container opening portion is capped in an annular space provided in the cap between a body portion and a fitting wall portion disposed on the inside of the body portion, whereby the cap is attached to the container opening portion (see Patent Document 1).
According to the related art, the annular space of the cap has a width in the radial direction that is slightly narrower than the thickness of the peripheral wall of the container opening portion. In this way, the cap is attached to the container opening portion by the mutual pressing force between the body portion and fitting wall portion and the container opening portion.
When the outer diameter of the container opening portion is substantially constant in a predetermined range in the axial direction, the related art has no problems. However, when the container opening portion is configured with a plurality of opening portions (step portions) with different outer diameters successively disposed in the axial direction, the related art results in a problem of sealing between the container opening portion and the cap.